Purpose: Colors spreading on inhomogenous visual field have often been considered as noise in the previous experiments with the noise masking technique (Li & Lennie, 1997; Sankeralli & Mullen,1997). However, when the spatial frequency of the inhomogenous field is low these colors appear no more noise but may have a different effect on chromatic discrimination. Our aim is to investigate effects of the spatial frequency of chromatic modulation on chromatic discrimination mechanisms. Methods: The stimulus was a random-array of isoluminant chromatic patches displayed on a CRT monitor. The chromatic distribution of the patches was either along the r-axis or along the b-axis of the MacLeod-Boynton chromaticity diagram. The mean chromaticities of the patches were the equal energy white (r=0.692, b=0.016) and a purple (r=0.692, b=0.029). Chromatic discrimination experiments were carried out along 16 radial directions from the mean chromaticity. Four patch sizes were examined (uniform, 6, 14 and 46 minutes). Results: When the patch size was small discrimination thresholds increased along the same axis as the chromatic distribution of patches. Therefore, these two axes must be independent and the color opponent mechanisms may be responsible for this discrimination task. The chromatic distributions of patches worked as noise. On the other hand when the patch size was large the r-axis and the b-axis chromatic discrimination were affected by the chromatic distributions along the orthogonal axes. These effects cannot be accounted for by the same discrimination model for small patch size, which indicates that there must be a higher order chromatic discrimination mechanism for large patch size. Conclusions: The visual system can use the color opponent mechanisms to perform chromatic discrimination when inhomogeneity is of high spatial frequency and regarded as visual noise, but it may use a higher order chromatic mechanisms for low spatial frequency inhomogeneity.